The invention relates to a projection exposure for forming fine patterns on a variety of solid state devices.
Circuit patterns have been formed in ever finer sizes in order to increase the degree of integration of solid state devices such as LSIs and to increase the speed of circuit operation. A reduction projection exposure method that features mass-producibility and excellent resolution has been used widely for forming such patterns. According to this method, the resolution varies in proportion to the exposure wavelength and varies in inverse proportion to the numerical aperture (NA) of the projection optical system. So far, it has been attempted to increase the resolution limit by increasing the numerical aperture (high NA). This method, however, is approaching its limit due to a decrease in the depth of focus and difficulty in the design of lenses and lens fabrication technology. In recent years, therefore, attention has been given to the approach for shortening the wavelength of the exposure light.
When use is made of vacuum ultraviolet rays such as from laser beams or soft X-rays such as SR light (synchrotron radiation light) as a source of light of short wavelengths, there is available no material that is adapted to the refractive optics (lenses) used thus far, and it becomes necessary to employ a reflection optical system. From the standpoint of precision and mechanical durability, a reflection mask is preferred to a transmission mask. The reflective reduction projection exposure method using vacuum ultraviolet rays or soft X-rays has been discussed in, for example, materials of the Academy of the Japanese Association of Electric Engineering, EDD-90-40, 1990, pp. 47-54 and "Soft x-ray Projection Lithography", Ceglio N. M. et al, J. Vac. Science Technology, B8 (6), Nov/Dec 1990. p. 1325-1328.
There has been proposed a phase shifting method in order to further enhance the resolution limit of the conventional reduction projection exposure method using refractive optics. According to this method, phases of light beams passing through neighboring opening patterns are inverted relative to each other in order to nearly double the spatial frequency response of the optical system for the repetitive patterns. In this method, a phase inverting plate (phase shifter) having a suitable index of refraction and is suitable thickness is selectively provided at a predetermined opening portion of the transmission mask to impart an effective optical path difference to the light beam passing through the neighboring opening patterns in order to invert the phases of light beams. The phase shifting method has been discussed in, for example, IEEE Transaction on Electron Devices, Vol. ED-29, No. 12, pp. 1828-1835, 1982.